Electrocardiogram (ECG) measuring devices are primarily used for measuring and monitoring the cardiac function of a patient, and for this purpose the total electrical activity of all the heart muscle fibers is recorded, e.g., by way of at least two measuring electrodes, as a so-called ECG signal. In addition to the two measuring electrodes, a third electrode is conventionally used for potential equalization. This electrode is also called a right leg drive (RLD) electrode.
When differential physiological signals, such as ECG signals, are measured, various types of noise occur. A very frequent type of noise is common mode noise. This noise arises when noise signals are transmitted through the patient to the two measuring inputs and dissimilar conditions, such as different impedances and capacitances, occur at the measuring inputs, with the result that the noise signals are not suppressed.
Another type of noise is electrode-based noise, that is to say noise that does not pass through the patient like common mode noise but is produced by spontaneous changes at the boundary between the electrode and the skin.
The electrode-based noise that most frequently occurs in an ECG is produced by pressure on the electrode or by tension on a cable connected to the electrode. These procedures alter the spacing between the electrode and the skin, so that the electrical properties of the boundary between the electrode and the skin change too.
Moreover, noise may pass through as a result of ionization of an electrode. This phenomenon occurs, for example, when X-rays are used for the purpose of medical imaging or therapy.
One way of reducing noise resulting from ionization consists in reducing the resistance at the boundary between the electrode and the skin. If the boundary has small resistance values, the ionization will not have a major effect. For this reason, conventionally the attempt is made to optimize the boundary between the electrode and the skin appropriately. However, this is not always possible in all cases.
Moreover, there are methods for monitoring the measured signals by a so-called single-channel heartbeat recognition algorithm. However, even with this algorithm noise may not always be distinguished from irregular heartbeats, which occur particularly frequently in patients suffering from heart disease.